1. Field of the Invention
The invention relates to phase change devices (PCDs) used in logic and memory applications.
2. Description of the Related Art
A two-terminal phase change device (PCD) 10 is shown in FIG. 1, and consists of a heating element 14 connected to a control terminal B. Heating element 14 acts as both a heater and an electrically conducting node in thermal and electrical contact with phase change material (PCM) 12, which connects to another metal terminal A which is simply an electrically conducting node connecting the PCD 10 to other circuitry (not shown). An electrical schematic of PCD 10 is shown in the right-hand side of FIG. 1, with the heating element 14 being represented as a resistance Rheater, and the PCM 12 being represented as a resistance RPCM.
In operation, a programming pulse is applied to PCD 10 in such a way as to selectively create a high resistance state or a low resistance state in the PCM 12, as explained with reference to FIGS. 2 and 3A. In particular, phase change materials are a class of materials which can change phase from crystalline structures to amorphous structures or back when under different thermal treatments, supplied in this example by heating element 14 by way of control terminal B. When phase change material 14 is heated above its crystallization temperature and cooled down gradually, it tends to form a crystalline phase and exhibits low electrical resistance (SET). When the phase change material is heated above its melting temperature and cooled down abruptly, it forms amorphous phase and exhibits high electrical resistance (RESET). Essentially, the material operates as a programmable resistor with two distinct electrical resistance values. Phase change materials may contain atom elements in group 4, 5 and 6 such as Ge, As, Se, Te.
FIG. 3A is a graph of the temperature profile for crystalline and amorphous phase change. Ta and Tx are melting and transition temperatures. t1 and t2 are time control periods for amorphous and crystallize state formation.
The two-terminal device of FIG. 1 consists of a volume of phase change material contacted on one end by a low resistance metal and on the other end contacted to a higher resistance interface. In order to program the device an electrical current is passed through the higher resistance interface into the phase change material. The heat generated in the high resistance interface along with the current injected into the phase change material causes the phase change material to change state. The way in which the current is removed will determine the final state of the material. By rapidly quenching the phase change material the material will be left in an amorphous state. If the materials temperature is slowly brought through the phase transition region The material will be left in a crystalline state.
Phase change materials have found their applications in optical disk memory such as CD-RW and DVD-RW based on its optical index change properties between crystalline and amorphous phases. In optical disk memory applications, a laser beam is used to introduce heat into the materials to switch between crystalline and amorphous states which have different refractive index. In integrated circuit applications, electric current is used to introduce joule heating into the phase change memory to switch between crystalline and amorphous states which have different resistance.
A problem associated with the two-terminal device structure depicted in FIG. 1 is that the heating element must be incorporated at one end of the device, which means that the heating element is directly in the electrical path between the two terminals of the device which directly affects the electrical characteristics of the device. This causes contention between the electrical characteristics of the device during its programming state and its read or non-programming state. In order to Set or Reset the PCD 10, the joule heating via the heating element 14 is used to transition the PCM 12 to the appropriate temperature. This requires that a trade-off be made between the electrical resistance of the PCD 10 and the thermal resistance of the PCD due to the heating element 14. In typical circuit application, designers are accustomed to being able to adjust resistance by making the material in the conductive path large. In this case growing the conductive path would also mean changing the amount of joule heating per unit area that is applied to the PCM 12 given that the programming circuitry does not change. The conducting path through the device incorporates the heater element which inherently needs to be of high resistance in order to heat at low enough currents. This means that a substantially high series resistance is included in the path between the two nodes. Thus there are three variables with strong interdependence that can not be decoupled in the 2 terminal device: resistance in the on state, current, resistance in the off state (the last to relate directly to the heating of the Phase change material).
Another issue with two-terminal devices is that the circuitry needed to program the device is directly connected to one or both of the terminals of the device. This means that the control function that determines the state of the device is also part of the nodes that are used to read the device or in any other non-programming state. This also can add capacitive loading or current paths to the device that would be seen in normal operations at the two terminals.